From my tests on Ubuntu 11.10 strongSwan - http://www.strongswan.org/
is the only IPsec client that works fine connecting to Fortinet Fortigate firewall - http://hwellmann.blogspot.com/2010/08/ubuntu-vpn-client-setup.html

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The following keyword is not supported by strongSwan:

interfaces="ipsec0=eth0"

because the native Linux kernel does not have an ipsec0
interface and

esp=aes128-sha1-modp1536,3des-sha1-modp1536

is not a valid notation for IKEv1. If you want a DH-Group
different from the IKE Main Mode one you must define: